The cargo on the 11-day mission consists mainly of large-scale spare parts – the first of five trips between now and September 2010 dedicated to stuffing every nook and cranny on the station with hardware it will need to serve as a orbiting lab through at least 2015, and possibly 2020.

The one remaining construction task comes in February, when one last "shirt-sleeve" module, "Tranquility" and its multi-windowed cupola, ride up on the shuttle Endeavour for installation.

Yet the realization that the end is near for the venerable shuttles, which NASA plans to retire in September 2010, also is sinking in.

For the past 28 years, and through two fatal accidents, shuttle crews have launched spectacular unmanned missions to Jupiter and Saturn. The orbiters have served as early versions of an orbiting lab with Spacelab and other research platforms. Crews have has launched, healed, then maintained the Hubble Space Telescope – arguably NASA's and the European Space Agency's most effective cosmic ambassador. And the orbiters have served as an international gateway through which the Japanese, Europeans, and other international partners have taken their initial footsteps in human spaceflight.

"We were talking in the firing room today about the number of missions to go, and the number per vehicle to go, and it's starting to hit home," acknowledges shuttle launch director Michael Leinbach.

Yet he quickly adds that, despite the poignancy of these final missions, and the uncertainty surrounding the future of the human-spaceflight program, a recent survey of employees indicated that 85 to 90 percent intend to stay on through the last shuttle flight.

In the meantime, the current mission is destined to keep people from dwelling on the program's end too much between now and the orbiter's scheduled return on Nov. 27. During the shuttle's stay, astronauts will conduct three spacewalks to help stow major spare parts along the station's trusses, as well as perform maintenance and small-scale set-up tasks for hardware on the station's exterior.

One piece of station hardware the crew will have to contend with: the sanitation system's urine processor, which is designed to convert urine into drinkable water. The ability to recycle as much liquid on the station as possible frees up weight on unmanned Russian, European, and Japanese cargo vessels for other space-station needs.

Beyond the space station, such systems will be mandatory for any human outposts spacefaring nations try to set up on the Moon or Mars.

Mission managers aim to bring back the urine processor's balky parts on Atlantis, then send replacement parts up on Endeavor in February.

After this mission ends, every new space-station crew member will have to hitch a ride on a Russian Soyuz rocket. This bridges the gap between the end of the shuttle program and the first flights of its replacement (currently, NASA's Ares 1 rocket). It also allows the shuttles to take more cargo uphill by reducing the amount of supplies needed to support a crew of seven.

More "up mass" also can be freed up by keeping the missions as short as possible given the tasks, which reduces the mount of "consumables" the craft must carry for its systems.